1. Field
This invention relates in general to power generation systems employing a Brayton cycle, and more particularly to using supercritical carbon dioxide (sCO2) as a working fluid.
2. Related Art
Power generation using the Brayton cycle with supercritical CO2 as the working fluid is currently being explored. Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. Carbon dioxide usually behaves as a gas in air at standard temperature and pressure, or as a solid called dry ice when frozen. If the temperature and pressure are both increased from the standard temperature and pressure to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, supercritical carbon dioxide behaves as a supercritical fluid above its critical temperature (304.25 K) and critical pressure (72.9 atm or 7.39 MPa), expanding to fill its container like a gas, but with a density like that of a liquid.
The Brayton cycle is a thermodynamic cycle using constant pressure, heat addition and rejection. Fuel and a compressor are used to heat and increase the pressure of a gas, i.e., the working fluid; the gas expands and spins the blades of a turbine, which, when connected to a generator, generates electricity. Power generation using a supercritical CO2 Brayton cycle system requires a recuperator to transfer heat from a lower pressure stream into the high pressure stream. Typically a recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply or exhaust air streams of a gas handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. Simple recuperation in the form of a counter-flow heat exchanger cannot perform this function efficiently in a Brayton cycled application using sCO2, because of the variable thermal properties of sCO2 in the two streams, i.e., the stream returning from the turbine to the compressor and the stream returning from the compressor to the heat source. The heat capacity off of the two streams varies with temperature in such a way that a mismatch exists that creates a large temperature difference in the recuperator. This causes a loss of thermal efficiency. Various remedies exist to split the cold stream flow, using multiple recuperators and multiple compressors, but these all increase the system complexity and suffer some loss in thermal efficiency.